Polysilazanes which provide easiness in molding works can easily give ceramic molded articles having various shapes when used as a raw material for molding.
Since a dense, high purity silicon oxide film (amorphous SiO.sub.2) is obtainable when an organic solvent solution of a polysilazane is used as a coating liquid and is calcined in air for reaction with water and oxygen, the polysilazane with such characteristics is practically used as a high purity silica coating material
Various methods are conventionally known for the synthesis of polysilazanes. For example, the following methods are known:
(1) A method for the synthesis of a polysilazane in which a silicon halide such as SiCl.sub.4 or SiH.sub.2 Cl.sub.2 is reacted with an amine (Japanese Published Unexamined Patent Applications No. Sho-60-226890, No. Sho-62-290730 and No. Sho-63-309526 and Japanese Examined Patent Publications No. Hei-2-15577 and No. Hei-6-62775, etc.); PA1 (2) A method in which a silazane is converted into a polysilazane using an alkali metal hydride catalyst such as KH having dehydrogenation activity (J. of Faint Technology Vol. 42, No. 543, April 1970, Japanese Published Unexamined Patent Applications No. Sho-60-226890, No. Sho-62-290730 and Sho-63-309526 and Japanese Examined Patent Publications No. Hei-2-15577 and No. Hei-6-62775, etc.); and PA1 (3) A method for the synthesis of a silazane by dehydrogenation of a silane compound with an amine compound using a transition metal complex catalyst such as Ru.sub.3 (CO).sub.12 (organometallics 1986, 5, 2081-2086, etc.). PA1 (4) A method in which an aminosilane is subjected to amine inter-exchange with ammonia using an acid catalyst such as CF.sub.3 SO.sub.3 H (Japanese Published Unexamined Patent Application No. Sho-62-225534 and Japanese Examined Patent Publication No. Hei-4-60492); and PA1 (5) A method in which an aminosilane is subjected to amine inter-exchange with a large amount of ammonia or an amine (Japanese Examined Patent Publication No. Hei-5-60488).
With method (1) above, however, an ammonium salt of hydrogen halide acid is produced as a by-product in a larger amount than the polysilazane, so that a large amount of the polymer is lost during the removal of the by-product. With method (2) above, it is necessary to take great care of the handling of the metal hydride catalyst which is dangerous, so that the method is ill-suited for a large scale synthesis. With method (3) above, it is very difficult to separate, after the reaction, the synthesized polymer from the transition metal complex catalyst such as Ru.sub.3 (CO).sub.12 which is very expensive and which is a homogeneous catalyst.
To avoid the use of a catalyst which generates a large amount of a by-product, which is dangerous or which is expensive, the following methods have been proposed in which an aminosilane is reacted with ammonia:
In the above method (4), however, since the ammonia as well as the amine liberated as a result of the amine inter-exchange are basic, the acid catalyst used reacts with the basic substances, so that the effect of the acid is diminished, resulting in a considerable reduction of the yield of the polymer. With the above method (5) using a large, excess amount of ammonia or an amine, a greater part of the ammonia and amine is discharged from the reaction system without being utilized for the synthesis, so that the method is disadvantageous from the standpoint of costs.